1. Field of the Invention
This invention relates to the control for controlling the bonding load of a fine lead electrode, and more particularly to the load correction effected when fine lead electrodes with a pitch of 80 .mu.m or less are bonded to bumps.
2. Description of the Related Art
As the size of the inner lead connected to a semiconductor chip becomes smaller, the lead width thereof becomes smaller. However, the lead is processed by the wet etching process and it is relatively difficult to enhance the lead processing precision. For example, in the case of 80 .mu.m pitch, the actually obtained final dimension of the lead width is 40.+-.5 .mu.m when the designed lead width is 40 .mu.m, and in the case of 60 .mu.m pitch, the finally obtained dimension of the lead width is 25.+-.5 .mu.m when the designed lead width is 25 .mu.m. Therefore, even if the lead pitch is reduced to 20 .mu.m and the lead width is reduced to 15 .mu.m, the lead processing precision is kept at .+-.5 .mu.m and is not enhanced. As a result, the processing becomes extremely difficult. Particularly, in the case of 50 .mu.m pitch, the designed lead width is 15 .mu.m and the actual dimension of the lead width is 15.+-.5 .mu.m so that the maximum value of the final dimension will be twice the minimum value thereof.
FIG. 1 is a flowchart showing the load control effected at the time of bonding according to an assembling method of the conventional semiconductor device. In this example, the bonding is effected by use of the TAB (tape automated bonding) technique. The positions of a carried TAB tape and semiconductor chip are recognized and aligned with each other, and the leads on the TAB tape and the bump electrodes on the chip are simultaneously bonded together. After the bonding is completed and they are carried, a next carried tape and chip are subjected to the same operation as that described above.
Conventionally, the load at the time of bonding is calculated based on the pressure for unit area necessary for the surfaces of the bump and lead to be connected together. The lead width used for the calculation was the designed value, that is, the central value of the final dimension. For example, if the pressure for unit area is 50 MPa and the area in which the bump and the lead with a pitch of 80 .mu.m are connected is 40.times.70 .mu.m.sup.2, the load for each lead becomes approx. 14 g.
If the load of 14 g is set, the pressures for unit area for the lead width 35 .mu.m (minimum final width) and 45 .mu.m (maximum final width) at the time of bonding respectively become 56.25 MPa and 44.25 MPa and the precisions thereof with respect to that of the final central lead width vary by .+-.12.5%.
Thus, in the case of 80 .mu.m pitch, even if the width of the lead whose designed dimension (central value of the final lead dimension) is 40 .mu.m is deflected by .+-.5 .mu.m, the unit area pressure is deflected only by .+-.12.5% and a problem that the lead is not bonded to the bump or the lead is pushed into the bump to expand the side portion of the bump to a large extent does not occur. However, in the cases of the lead pitches of 60 .mu.m and 50 .mu.m, the deflection amounts of the unit area pressures for the respective final central lead widths are .+-.20% and .+-.33%, the ratios of the maximum values of the lead widths to the minimum values are 1.5 and 2.0, the ratios of the unit area pressures applied to the maximum lead widths to the unit area pressures applied to the minimum lead widths are respectively 1.5 and 2.0, and a problem that the lead is not bonded to the bump or the lead is pushed into the bump to expand the side portion of the bump to a large extent and cause the adjacent bumps to be short-circuited may occur.
The above problem is explained below with reference to FIGS. 2A, 2B, 3A and 3B. FIGS. 2A to 3B are cross sectional views showing the states in which leads 2 are bonded on respective bumps 3 on a semiconductor chip 4 by use of a bonding tool 1. For example, in a case where the leads are arranged with a pitch of 50 .mu.m and when the pressure for unit area is 50 MPa and the area in which the lead with the final central lead width is connected is 15.times.70 .mu.m.sup.2 the load for each lead becomes approx. 5 g. When the load of 5 g is set, the pressures for unit area for the final lead widths 20 .mu.m and 10 .mu.m at the time of bonding respectively become 33.5 MPa and 66.25 MPa. In FIGS. 2A and 2B, the states set at the time of bonding of the lead with the lead width of 20 .mu.m are shown and in FIGS. 3A and 3B, the states set at the time of bonding of the lead with the lead width of 10 .mu.m are shown. The length of an arrow 5 indicates the magnitude of the load applied to the lead of unit area and the same load is applied to the lead in FIGS. 2B and 3B. In the above condition, the pressure of 33.5 MPa for unit area is applied to the lead of width 20 .mu.m to effect the bonding and the pressure of 66.25 MPa for unit area is applied to the lead of width 10 .mu.m to effect the bonding.
In FIG. 2B, the state in which irregular surface portions of the surfaces of the bump and lead cannot be made flat and sufficient bonding cannot be attained since the pressure for unit area is insufficient is shown. In FIG. 3B, the state in which the adjacent bumps may be short-circuited since the pressure for unit area is excessively large and the lead is forcedly pushed into the bump to expand the side portion of the bump is shown.
Thus, the width of the processed lead varies within a permissible range, but the conventional method in which the load for unit lead used at the time of bonding is set based on the central dimension (standard dimension) which is the designed value prior to the processing cannot cope with the present situation in which the lead pitch is increasingly reduced and causes a problem that the yield is significantly lowered.